I learned this raku technique from a fellow potter/sculptor, Kathy Mathisen. This glaze is a perfect example of how copper reacts with the change in atmosphere during the raku process. Copper, under complete reduction, turns copper red. When little oxygen is introduced during a process called "burping", copper red turns into a spectrum of other colors. Too much oxygen turns it into a dull green.
Ok now i'm gonna try to explain how tricky this glaze can be. First of all this glaze consist of 80% copper carbonate and 20% frit, the high density of copper in this glaze, gives its richness. and longer "shelf life"(will explain later).. The downside to this glaze is that it is so sensitive to the atmosphere during the raku process. It also requires a few extra steps to get it just right. Unlike most glazes, this glaze should be sprayed on a bone dry
(fingernail thickness) instead of bisque. The reason for this step is to mature the glaze with the piece during a bisque firing.
Why can't the glaze be matured during a Raku firing (cone 06)? This glaze does not need to get to that hot temperature for the copper to react with reduction. In fact the piece will be too hot. The copper will reduce and turn copper red, however if you burp it when the piece is too hot, the heat will not be able to freeze that change in color. The piece will just burn any oxygen you introduce. Now if you do this technique in a "colder" firing, (cone-018) instead of 06 ,almost a 5oo degree difference, the piece won't be hot enough to burn the oxygen.
Why can't I spray the glaze on bisque and then raku fire it @ cone 018? If you don't mature the glaze at cone 06 the frit (glass) in the glaze won't flux(melt) the glaze then would simply flake of the pot and can easily be rubbed off.
This technique actually saves you propane, since you don't have to heat the raku kiln to 06 just to mature the glaze. bisque firing will take care of that.
Ok so you fire this glaze to cone o18. Pull the piece out of the kiln, then placed in a container filled with combustible material, cover quickly. wait a few minutes depending on the piece. you have to let the piece cool a little in the container before re oxidizing(burping)
Form, thickness of the piece, can be a factor . Certain forms, like closed forms hold in heat much longer, there for the time it takes from reduction to re oxidation should be lengthen. Open forms cool much quicker, there fore should be re oxidized much sooner. Same goes with the thickness of the piece. The thicker it is the more time it takes for the piece to reach the proper temperature for re oxidizing. Trial and error. that's the game. Bracketing is one way to achieve success. Start by firing 3 identical pieces. First piece wait, 2 minutes before burping, the next piece, wait 4 minutes, and 6 minutes on the third. wait about half hour before opening the container to completely cool the piece in the container to prevent to much oxidation, this will let the glaze color freeze in place. Then analyze the results
Another downside to this glaze is that it is a temporary glaze. What i mean is that the copper in this glaze will continue to react with the air around it, like a penny or the statue of liberty.
this is the "shelf life" of the glaze. I found that copper matte glazes has a longer shelf life than copper luster glazes. Luster glazes re oxidize in less than a year, while copper matte glazes has about ten years before it completely re oxidizes. I think the higher the density of copper matte glaze (80%) in the glaze makes a difference. Luster glazes usually contain less than 10% copper carbonate.